7-33 Rare Earth Element Concentrations and Their Relation to Marine Environments at the Mt. Simon - Eau Claire Contact

Session: Undergraduate Research, Part I (Posters)

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The stratigraphy of the Mt. Simon and Eau Claire Formations in the midwestern United States is widely documented in the literature; however, the rare earth element concentrations (REEs) in these units have not been studied. The middle to upper Cambrian strata of the Mt. Simon–Eau Claire Formations in Iowa host anomalous concentrations of phosphate-rich (as high as 14.2% P2O5) rock, along with high concentrations of REEs, reaching 2,170 ppm. This phosphogenesis event occurs at the transition between the Mt. Simon and the Eau Claire Formations, which coincides with the Sauk II transgression in the upper mid-continent United States, indicating high marine primary productivity. Phosphorites and ironstones were sampled from eight different cores in the repository at the Iowa Geological Survey through ICP-OES, WDXRF, and pXRF. The cores are widely distributed across Iowa and represent a transition from intertidal to deeper shelf environments. The areas with the highest REE concentrations include north central, northeast, and southeast Iowa. Phosphatic conglomerate and brachiopod samples (n=21) have an average total REE concentration of 1,132 ppm, dominated by light REE (average LREE fraction 0.73) and an average phosphate content of 8.0%. Oolitic and pisolitic ironstone samples (n=20) only occur in northern Iowa and show low average total REE (523 ppm) and phosphate (4.2%) contents, but have a higher average heavy REE fraction (0.32) compared to samples from phosphatic brachiopods and conglomerates. Only Ce, La, and Nd had concentrations over 100 ppm in each sample at each core. Data with the pXRF show cycles of higher REE concentrations through the depth of the cores, hinting at cycles of upwelling and changing redox conditions. The origin of these deposits is being investigated. HREE concentrations in shallow, near-shore environments supports a source rock origin, but episodes of seafloor spreading in the Iapetus Sea, along with cycles of upwelling, could have introduced REE heavy seawater into near shore areas. 

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